Firefox: One of the greatest things about Firefox is its customizability. While you can do a lot with userscripts and userstyles, free extension Mar Mod packs in a ton of useful tweaks into one, simple package, including quick access to any of your add-on settings, favicon customization, and one-click private browsing.

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Mar Mod isn't like other extensions. Instead of just adding one useful feature to Firefox (or numerous features within a certain category, like Tab Mix Plus), it goes around and fixes a number of tiny annoyances, including ones you didn't even know you had. Some of my personal favorite additions include:

Lets you access the settings window for any of your add-ons through the Firefox menu

Resizes the Firefox Button and puts it next to your tab bar

Adds a "Restart" button to the Firefox menu

Lets you change the favicon of any bookmark

Provides a "Private Browsing" button that you can add to the toolbar

Simplifies the add-on page, shrinking each row down so you can view all your add-ons at once (and click on them for more info)

Translates a whole page or selected text from the context menu

It does even more than this, so you can check out its add-on page for more info. The only annoying thing is that it enables many of its additions by default, so when you first install it, you may encounter as many new annoyances as you fixed, since a lot of small things will work differently. I highly recommend combing through its preferences and turning off anything you don't want, since there are bound to be a few features in there you'd rather keep default. Other than that, though, it's a really great add-on, and provides a lot of nice little fixes and fills in some of Firefox's missing features.